Membranes and polymers have been used to separate, remove, purify or partially recover a variety of components from mixtures, e.g., gases including hydrogen, helium, oxygen, nitrogen, argon, carbon monoxide, carbon dioxide, ammonia, water vapor, methane and other light hydrocarbons. Generally, this separation is dependent on the permeability and the diffusion of the molecules through the polymer. For example, one of the components may selectively permeate the polymer and/or diffuse through the polymer more readily than another component of the mixture; whereas a relative non-permeating component passes less readily through the polymer than other components of the mixture.
The separation of diffusants (e.g., molecules or compounds) using a polymer is dependent on both the polymer and the diffusants. Therefore, there are many factors that influence diffusion, including: (1) the molecular size of the diffusant; (2) the physical state of the diffusant; (3) the composition of the polymer; (4) the morphology of the polymer; (5) the compatibility of the polymer and the diffusant; (6) the solubility limit of the diffusant within the polymer matrix; and (7) surface or interfacial energies of the polymer.
Diffusivity plays a role in the separation of the gases in a mixture and can be thought of on a simple level as relating to the size of the molecules diffusing through the polymer. Smaller molecules can more easily penetrate and diffuse through a polymer matrix. The separation of diffusants is also based on the relative permeability of the diffusant through the polymer. Permeability is a measure of the rate at which a particular gas moves through a membrane of a standard thickness under a standard pressure difference. Permeability depends both on the solubility of the permeating gas in the polymer and its diffusion coefficient. The diffusants contact one side of a polymer, which is selectively permeable, allowing the one diffusant to pass through the polymer more readily than another diffusant. The difference in permeability of the diffusants allows a diffusant to be separated when an appropriate membrane or polymer is selected.
Diffusivity is also dependant on the repulsive component of the interaction between molecules and the polymer. The free volume or the unoccupied volume of the polymer also plays a role in the diffusivity, with molecules diffusing more easily through a polymer with a higher free volume. Additionally, the polymer with a higher unoccupied volume often results results in less discrimination between diffusants on the basis of molecular size of all gases. The dynamics of the motion (e.g., rotation and vibration) of the subunits of the polymer affect the spacing between polymer subunits and thus influence the separation. Although unoccupied volume in a polymer is important in determining membrane separation characteristics, other factors are very significant in achieving improvements in such characteristics. These motions prevent the polymer from maintaining optimum spacing between polymer subunits for the desired separation. Generally, lower temperatures will reduce the frequency and amplitude of motions by the polymer matrix and thus affect the separation of the components.
The separation is dependent on other factors as well, e.g., the temperature, the specific properties of the membrane and the properties of the component gases of the gas mixture to be separated. Often, the permeability of a membrane to a gas decreases as the temperature decreases, whereas, the separation factor increases as the temperature decreases. Thus, in many instances the temperature is maintained at a relatively high level to increase the rate of gas permeation through the membrane; however, in some instances the temperature is maintained near or below ambient temperature.
Currently, stiff-chain, rigid, glassy polymers, rubbery polymers and elastomeric polymers have been used to separate mixtures of gas. Stiff-chain, rigid, glassy polymers (e.g., polysulfone, cellulose acetate and polyimide polymers) used for separation of gases and the gas diffusivity play a dominate role in the separation and the ability of gas molecules to permeate is size dependent. In glassy polymers, smaller gas molecules such as helium and hydrogen are more permeable than larger molecules such as oxygen, nitrogen and methane. However, because they are rigid and inflexible, glassy polymer membranes are typified by low fluxes, while rubbery or elastomeric polymers have polymer chains that are more flexible and less discriminating based on diffusant molecular size, and diffusant solubility effects can play a dominant role in selectivity. The flexible polymer chains are relatively permeable to many gases, but are often not very selective for one gas over another. Generally, permeability for rubbery polymers is much greater than for more rigid glassy polymers. Consequently, prior-art gas separation membranes tend to exhibit either high gas permeation rates at the sacrifice of high permselectivity or the inverse.
Currently in the art, it is not possible to predict the gas selectivity or the intrinsic permeability of a polymer for given gases under a given set of conditions from knowledge of the selectivity of another pair of gases, even under the same conditions (e.g. temperature, pressure) as it is dependent on the structure of the polymer, the morphology of the membrane, the gas composition and properties. The gas selectivity and permeability must be determined experimentally.
FIG. 1 is a graph comparing a filled rubbery polymer to the theoretical value given by Maxwell's model. The pure CH4 permeability in filled natural rubber polymers is plotted as a function of ZnO particle concentration in the polymers and compared with the theoretical predictions given by Maxwell's model for spherical particles shown below.
            P      C              P      P        =      (                  1        -                  ϕ          f                            1        +                              ϕ            f                    2                      )  Where φf is the volume fraction filler. Pc and Pp are the component permeability and the polymer permeability respectively. In FIG. 1, the dashed line represents value calculated by Maxwell's model for permeability in a permeable matrix filled with an increasing amount of impermeable, spherical particles (Barrer et al., J. Polymer Science, Part A: Polymer Chemistry, 1 (1963) 2565-2586). The graph illustrates the permeability as a function of particle concentration in the polymer, and shows a departure from the value predicted by Maxwell's model.
Other studies in the art have shown that loading polymers with various particles result in a reduction in the permeability, as seen in the table 1 below:
Particle LoadingPermeabilityPolymerParticle(vol. %)Reduction (%)PDMS1Graphite634% (N2)SBR2Carbon Black2339% (N2)Polyester-amide3Organoclay37.580% (O2)1Lape et al., J. Membrane Science, 236, (2004), 29-37;2Wang et al., Polymer, 46 (2005), 719-724; and3Krook et al., Poly. Eng. And Sci., (2005), 135-141.
Still others have examined the addition of particles to polymers to alter gas separations but have actually seen a decrease in the permeability with increasing particle concentration. In spite of the considerable research effort in separation membranes and polymers there has been limited advances in gas separations. Furthermore, improvements in selectivity for one gas over another are generally obtained at the expense of permeability.
The foregoing problems have been recognized for many years and while numerous solutions have been proposed, none of them adequately address all of the problems in a single device, e.g., selective gas separation with improved permeability.